Use is_zero workaround on clang

The same warning appears on clang for windows, but the workaround wasn't
used because the gcc macro wasn't defined.

include/boost/functional/hash/detail/hash_float.hpp

@@ -241,7 +241,7 @@ namespace boost
         template <class T>
         inline bool is_zero(T v)
         {
-#if !defined(__GNUC__)
+#if !defined(__GNUC__) && !defined(__clang__)
             return v == 0;
 #else
             // GCC's '-Wfloat-equal' will complain about comparing